(1) Field of the Invention
This invention relates to a catalyst for trimerization of ethylene and a process for trimerizing ethylene using the catalyst. More specifically, it relates to a catalyst exhibiting an enhanced activity for trimerization of ethylene to produce 1-hexene, which is used as a comonomer for the production of linear low-density polyethylene (LLDPE), and further to a process for trimerizing ethylene by which 1-hexene can be produced effectively and highly selectively.
(2) Description of the Related Art
It is known to use a chromium compound as a catalyst for trimerization of ethylene to give 1-hexene. For example, a catalyst system comprising a chromium compound, polyhydrocarbylaluminum oxide and a donor ligand is described in Japanese Unexamined Patent Publication No. (hereinafter abbreviated to “JP-A”) S62-265237. A catalyst system comprising a chromium compound, a pyrrole-containing compound, an alkyl metal compound and a halide is described in JP-A H6-239920. A catalyst system comprising a chromium compound, an alkyl metal compound, and an acid amide or imide compound is described in JP-A H8-59732. A catalyst comprising (i) a complex of chromium salt with a multidentate ligand selected from phosphine, arsine and stibine, and (ii) aluminoxane is described in JP-A H6-298673. A catalyst comprising (i) a chromium-chlorine complex or alkyl chromium complex having a specific nitrogen ligand and (ii) an aluminum compound is described in JP-A H10-7712. A catalyst comprising (i) a chromium complex having a cyclic polyamine or hydro-tris(pyrazolyl)borate ligand and (ii) an alkyl aluminum compound is described in JP-A H10-231317.
However, these chromium catalysts have problems as explained below.
When the catalyst of JP-A S62-265237 is used for trimerization of ethylene, a large amount of polyethylene is produced in addition to 1-hexene. When the catalyst of JP-A H6-239920 is used, the amount of polyethylene produced can be reduced. However, a pyrrole-containing compound, which is one ingredient of the catalyst, is extremely unstable to air, and readily deteriorated and colored. Thus, a pyrrole-containing compound is troublesome to handle, and a treating process or apparatus for removing a coloring matter from the catalyst or purifying the catalyst is needed.
As for the catalyst of JP-A H8-59732, among the acid amide or imide compounds, which are one ingredient of the catalyst, maleimide is optimum for the catalytic activity for trimerization of ethylene. However, maleimide has problems such that it has a poor solubility in an organic solvent and the catalyst is troublesome to prepare, and further that it is not readily commercially available and is expensive.
The catalyst of JP-A H6-298673 has a problem such that it is difficult to carry out a process for trimerization of ethylene with a good reproducibility. The catalyst of JP-A H10-7712 has a poor activity for trimerization of ethylene. The catalyst of JP-A H10-231317 has problems such that a large amount of polyethylene is undesirably produced, and that the selectivity to 1-hexene among oligomers is low.